This invention relates to a refrigerating apparatus such as a household refrigerator provided with a noise attenuating function actively attenuating noise produced from a compressor of the refrigerator or the like.
Almost every home is generally furnished with a refrigerating apparatus employing a compressor, for example, a household refrigerator. Since such a refrigerator is in continuous operation throughout the year, it is important to solve a problem of noise produced therefrom. In the refrigerator, one critical noise source is a machine compartment enclosing a compressor and piping system connected to the compressor. More specifically, from the machine compartment emanates a relatively loud noise, for example, a noise produced from driving of a compressor motor, noise produced from the flow of a compressed gas and mechanical noise produced by moving members of a compression system. Furthermore, the piping system connected to the compressor produces noise due to vibration thereof. The noises emanating from the machine compartment thus account for a large part of the noise of the refrigerator. Accordingly, control of the noise from the machine compartment contributes to noise reduction in the refrigerator.
Conventionally, compressors of the low noise type such as a rotary compressor have been employed for the purpose of reducing the noise emanating from the machine compartment. Further, the construction of vibration-proofing of the compressor has been improved and the configuration of the piping has been improved, thereby providing damping of the vibration in a vibration transfer path. Further, noise absorptive and insulative members have been disposed around the compressor and piping system, thereby improving an amount of noise absorbed in the machine compartment and a noise transfer loss.
However, a plurality of ventilating openings are formed in one or more walls defining the machine compartment for ventilating the machine compartment, and the noise produced in the machine compartment leaks outward through the ventilating openings. As the result of the provision of the ventilating openings, the above-mentioned conventional noise-reduction methods each have a definite limit and provide at most noise reduction of 2 dB.
With the advancement of applied electronic techniques including sound data processing circuitry and acoustic control techniques, application of an active noise control system wherein noise is attenuated by the effect of sound wave interference has recently been taken into consideration. More specifically, in the above-mentioned active noise control system, detection means such as a microphone is provided at a specific position in the machine compartment for receiving sound emanating from a noise source and converting the received noise to a corresponding electrical signal. The electrical signal is then processed to a cancellation signal by an operational unit. The cancellation signal is supplied to a cancellation sound producer such as a speaker so that an artificial cancellation sound of opposite phase or 180.degree. out of phase with the noise received by the microphone and having the same frequency and amplitude as that of the received noise is produced by the speaker, so that the artificial sound interferes with the received noise, thereby attenuating the noise.
When the above-described active noise control system is put to a practical use, it is necessary to compensate for variations of characteristics of a noise attenuating signal system due to both aged deterioration of parts composing the signal system and the ambient temperature. For this purpose, it is proposed that an operational factor or acoustical transfer function be compensated for in accordance with variations of the noise attenuating capability of the active noise control system. To perform such a compensation, it is proposed that a noise attenuation monitoring sound receiver such as a microphone be provided for monitoring a sound attenuation effect of the control sound producer and that control means is provided for changing the operational factor of the operational unit by a predetermined amount when the monitoring result shows that the operational factor is out of a predetermined tolerance. The control means is adapted to continuously perform the operation of changing the operational factor until the operational factor comes into the tolerance. Such a control is referred to as an adaptive control wherein the noise attenuation effect in the active noise control is maintained at an optimum level.
To perform a desirable adaptive control, the noise attenuation monitoring sound receiver needs to be disposed away from the cancellation sound producer accurately by a preselected distance. Actually, however, variations in the distance between the monitoring sound receiver and the cancellation sound producer during assembly steps, which reduces the accuracy of the adaptive control. When the assembly accuracy is improved such that the variations in the distance between the monitoring sound receiver and the cancellation sound producer can be ignored, the accuracy of jigs used to mount the receiver and producer needs to improved and a careful assemblage is needed, resulting in lowered working efficiency and increased production cost.